Adds description for the demo app in the README

Change-Id: Ia4c7519316ec399ae12ef615f8f2950d2435fbaf

README.md

@@ -229,6 +229,13 @@ equivalent attribute
   but as explained above, Android developers will benefit by having this attribute for having
   more control over when a wrapping happens.
 
+## Flexbox Playground demo app
+The `app` module works as a playground demo app for trying various values for the supported attributes.
+You can install it by
+```
+./gradlew installDebug`
+```
+
 ## How to make contributions
 Please read and follow the steps in [CONTRIBUTING.md](https://github.com/google/flexbox-layout/blob/master/CONTRIBUTING.md)